1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and more particularly to a transflective liquid crystal display device having a reflective display mode in which ambient light is selectively reflected and a transmissive display mode in which backlight is selectively transmitted.
2. Description of the Related Art
Liquid crystal display devices have widely been applied to various technical fields by virtue of their features such as light weight, small thickness and low power consumption.
The liquid crystal display device has been required to eliminate a difference in appearance of the display screen due to the environment of use, in particular, ambient brightness. For example, in order to optimize the brightness of transmissive display in a dark place, a technique has been disclosed to provide a device which measures the amount of ambient light, adjusts the luminance of an illumination light source in accordance with the measured result, and effects easy-to-view display with a proper luminance and a less amount of electric current consumed (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 6-18880). In addition, in order to improve degradation in gradation of reflective display in a light place, a technique has been proposed to partially thin out video signal bits in accordance with the intensity of ambient light, and to reduce the number of signal bits that are used, thereby limiting the number of gradation levels of display and increasing a difference in luminance between gradation levels (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 10-282474).
In recent years, attention has been paid to a liquid crystal display device which uses an optically compensated bend (OCB) alignment technique, as a liquid crystal display device which can realize an increase in viewing angle and response speed. The OCB mode liquid crystal display device is configured such that a liquid crystal layer including liquid crystal molecules, which are bend-aligned, is held between a pair of substrates in a state in which a predetermined voltage is applied. Compared to a twisted nematic (TN) mode, the OCB mode is advantageous in that the response speed can be increased and the viewing angle can be increased since the effect of birefringence of light, which passes through the liquid crystal layer, can be self-compensated by the alignment state of liquid crystal molecules.
In addition, there has been proposed a transflective OCB liquid crystal display in which each of pixels includes a reflective display part and a transmissive display part.
In the transflective liquid crystal display device, a good display image can be obtained, regardless of the environment of use, by mainly executing, in a light place, display in the reflective display mode by the reflective display part, and by mainly executing, in a dark place, display in the transmissive display mode by the transmissive display part.
However, it is not possible to make the voltage-transmittance characteristics in the transmissive display part agree completely with the voltage-reflectance characteristics in the reflective display part.
In the above-described transflective liquid crystal display device, in general, a pixel electrode which constitutes the reflective display part is electrically connected to a pixel electrode which constitutes the transmissive display part. Thus, in the case where a voltage relative to an input gradation level is common between the transmissive display part and reflective display part, the transmittance characteristics (transmissive gamma) relative to the input gradation level in the transmissive display part do not agree with the reflectance characteristics (reflective gamma) relative to the input gradation level in the reflective display part.
Consequently, a difference occurs in appearance of the display screen due to the environment of use, in particular, due to ambient brightness. Specifically, in the case where the gamma characteristics are different between the reflective display part and transmissive display part, there arises a problem that the image quality of the display screen differs between a light place where the reflective display mode is a main mode and a dark place where the transmissive display mode is a main mode.